Many pumps operate intermittently in response to the level of the liquid being pumped. Examples of these pumps are bilge pumps and basement pumps.
Pumps of this type are commonly controlled by a float-type level controller which may actuate a mercury switch in response to the level of the liquid to start and stop the pump. These pump controllers require conductors from the switch to the pump motor, and the switch and conductors must function in an environment which may include a corrosive liquid, such as salt water and/or other contaminants of the type found in bilges, flooded basements, etc. The orientation of the switch controls the on-off action, and this is a disadvantage in marine applications in which the boat undergoes pitching or rolling movement.
It is also known to use a liquid level sensor which uses the pressure of a gas in a sensing chamber to determine liquid level. For example, as shown in Staak U.S. Pat. No. 2,804,516, a sensing chamber which is open to the liquid can be employed so that a gas, such as air, is compressed in the sensing chamber by the liquid. Thus, the pressure of the gas rises and falls, respectively, with rising and falling liquid levels, and this can be used to control operation of the pump.
One problem with using a sensing chamber to control the pump is in obtaining pump shut off when the liquid level drops to the desired shut-off level. More specifically, we have found that the column of water or air and water in the standpipe on the pump discharge provides sufficient pressure in the sensing chamber to preclude prompt pump shut off. In this regard, the rotating impeller of the pump serves as a valve to prevent the liquid in the standpipe from emptying by gravity back into the sump, and the pressure sensed by the sensing chamber is the full pump discharge pressure because it communicates directly with the periphery of the impeller where the pump discharge pressure is maximum. Although the pump will eventually shut off after a period of sputtering output, the pump runs unnecessarily long after essentially all of the liquid has been evacuated.